The present invention relates to a probe for measuring pressure oscillations, and in particular to probes for measuring pressure oscillations in combustors of gas turbines. The invention also relates to the use of probes according to the invention.
Pressure oscillations occurring in the combustors of modern gas turbines, so-called combustor pulsations or combustion pulsations, often also simply called pulsations, provide important indications of the quality of the combustion, especially when employing premix burner technology. Under unfavorable conditions, the combustor pulsations may reach amplitudes at which the mechanical integrity of gas turbine components is at risk. This means that a permanent monitoring of combustor pressure oscillations basically is now indispensable. Because of the high temperatures, a direct detection of occurring pressure oscillations requires high-temperature-resistant pressure sensors, which on the one hand are very expensive, and on the other hand are confronted with usage conditions that are so extreme that a significant probability of failure exists during continuous operation. It is also known that the sensor characteristic of such sensors is temperature-dependent, which also makes the quantification of the measured pressure oscillations harder or allows it only with limited accuracy. In order to not expose the sensors to excessive temperatures, they are set back from the combustor wall a distance by means of an adapter. However, such an adapter has a resonance behavior that influences the acoustic signal. Similar tasks for identical problems encountered in the realization of a measuring device naturally exist also in other combustors and hot gas flows.
For this reason, the use of so-called long-line probes is known. In these, the actual measuring point within the gas turbine combustor is connected by means of a line, basically by means of a small tube, with a pressure transmitter positioned outside of the combustor. This makes it possible that the transmitter can be used at substantially lower temperatures. For this reason, substantially cheaper pressure transmitters or microphones, whose useful life and measuring accuracy is not limited by extreme usage conditions, can be used. In such a configuration, it is important to ensure an echo-free termination of the measuring line formed in this manner, and, if possible, to also avoid any type of reflections within the measuring line.
The termination of the measuring tube with a semi-infinite tube is known. This is realized with a line having a long length. The line or semi-infinite tube is connected at a first end with the end of the measuring tube opposite from the end that faces the measuring point. With sufficient length, the pressure oscillations are attenuated inside the semi-infinite tube as a result of internal dissipation in such a way that no significant amplitude reflections remain at the second end of the semi-infinite tube. According to the state of the art, the second end of the semi-infinite tube is simply closed off in order to prevent hot gas leaks. The disadvantage of this is that the entire measuring device is filled with hot and aggressive combustion gases, and the transmitter is again exposed to elevated loads. Furthermore, conventional long-line probes with coupled semi-infinite lines are difficult to handle in practice, since the current state of the art does not offer any solution as to how to include the semi-infinite tube as an integral component of the probe. This means that the state of the art does not offer any solution, in which an easily manageable, robust, and compact probe is available for measuring pressure oscillations in combustors, in which probe the pressure transmitter is positioned at a distance from the actual, thermally loaded measuring point.
In view of the above-disadvantages with the prior art, an embodiment of the invention provides a so-called long-line probe for measuring pressure oscillations in combustors, with the long-line probe being easily manageable, compact, and robust. The probe according to the invention can be used in a frequency range from 0 Hz to 10 kHz without any significant falsification of the signals due to resonances that may occur. In the interest of better handling, the probe is preferably a compact embodiment. Any potentially necessary supply lines are integrally embodied in this probe in order to prevent a risk of damaging external connection lines as much as possible. The probe must be suitable for maintenance-free continuous operation of several tens of thousands of operating hours. Should any damage occur, a simple, quick replacement of the entire probe module must be possible.
According to a preferred embodiment of the invention, the probe includes the following elements:
an inner tube functioning as a measuring tube, with one end of the inner measuring tube being positioned on the measuring point side of the probe, and the opposite end of the measuring tube being positioned on the transmitter side of the probe;
an outer tube, which is positioned so as to envelop the measuring tube at least partially, and an outer wall of the measuring tube and an inner wall of the outer tube defining therebetween a toroidal space open to one side;
a pressure transmitter, which is in connection with the interior of the inner measuring tube in the area of the transmitter-side end of the measuring tube; and
a semi-infinite tube, which is connected at a first end to the transmitter-side end of the measuring tube, and which is connected at a second end to the toroidal space, the semi-infinite tube being constructed as a winding positioned around at least one of the measuring tube and the outer tube.
In one preferred embodiment, the inner measuring tube is provided at its outer wall with a thermal insulation. With the help of this measure, temperature gradients within the measuring tube that would influence the measuring result are avoided as much as possible.
The formation of the semi-infinite tube as a winding around an actual probe tube ensures a compact design. Furthermore, a robust connector for a flushing gas can be provided via the outer tube. This makes it possible to flush the semi-infinite tube and the measuring tube with a flushing gas, so that a penetration of combustion gases into the actual measuring technology is prevented. The flushing gas furthermore helps in preventing the occurrence of temperature gradients in the measuring tube.
In order to avoid undesired reflections, within the measuring tube, the length of the semi-infinite tube is preferably more than 7000 times its diameter. Advantageous embodiments of the invention have semi-infinite tube lengths of more than 40 meters, and even more preferably equal to or greater than 50 meters. In order to avoid interfering seams that again would result in reflections with echo effects, it is also advantageous in this connection that the semi-infinite tube has the same internal diameter as the measuring tube. These internal diameters are selected to be preferably in the range from 4 to 10 mm, even more preferably approximately 6 mm.
An echo-free, or at least, low-echo termination of the measuring device additionally can be improved, if so required, by providing an orifice at the second end of the semi-infinite tube. The diameter of the orifice is preferably selected in the range from 1.5 to 2 mm.
As already mentioned, it is advantageous to connect the toroidal space with a flushing gas supply. A permanent flushing gas supply is preferred. The flow of the flushing is preferably adjusted so that the flow velocity in the measuring tube is below 3 m/s.
The probe according to the invention is particularly suitable for use in gas turbines, wherein the measuring-point end of the measuring tube is open towards a combustor of the gas turbine. The toroidal space is preferably connected with the combustor plenum of the gas turbine. This ensures the flushing air supply as long as the gas turbine is operating, and the pressure of the flushing air is about 1 bar higher than the combustor pressure at the measuring-point end of the probe. This results in an inherently safe system, and the penetration of hot combustion gases into the probe, and thus any contact of hot gas with the pressure transmitter is reliably prevented. The flushing air is provided in this embodiment in a modern gas turbine at a temperature of about 350-400xc2x0 C. or even higher. It is especially advantageous that the entire configuration is then designed so that the flushing air is cooled when flowing through the semi-infinite tube to a range of slightly above 100xc2x0 C., for example, 120xc2x0 C. to about 200xc2x0 C. In a preferred embodiment, the flushing gas is at temperatures in the range from 150xc2x0 C. to 180xc2x0 C. by the time the flushing gas enters the measuring tube. This temperature range has the advantage that, on the one hand, condensation is prevented, but, on the other hand, a pressure transmitter, whose upper acceptable usage temperature is specified, for example, as 200xc2x0 C., can be easily used. For this purpose, the winding carrier can be provided with ventilation openings. These openings ensure that atmospheric air is able to flow through the winding and around the semi-infinite tube, so that medium flowing within the semi-infinite tube is cooled.
When used in gas turbines, it is known that the pulsation values measured with the probe are used for regulating and protection actions. This means that when an acceptable upper value is exceeded, an emergency shutdown or protective relief of the machine can be initiated, or, adjustments of certain combustion parameters, such as the control of premix burners or water injection, can be made in relationship to measured combustor pulsations. Naturally, the probe according to the invention also can be used very well for other combustion chambers and hot gas flows.